How it works
A proportion sets two ratios equal. Cross-multiplying turns that equality into a single equation, a×d = b×c, which rearranges to solve for whichever term you're missing.
a ÷ b = c ÷ d ⇔ a × d = b × c
a = (b × c) ÷ d · b = (a × d) ÷ c · c = (a × d) ÷ b · d = (b × c) ÷ a
Worked example
2/3 = c/12 — what is c?
- Cross-multiply: 2 × 12 = 24.
- Divide by the remaining term: 24 ÷ 3 = 8.
c = 8, so 2/3 = 8/12.
Common questions
What's a real-world example of a proportion?
Scaling a recipe: if 2 cups of flour makes 3 dozen cookies, how much flour makes 12 dozen? Set up 2/3 = c/12 and solve for c — exactly this calculator.
Does the order of a, b, c, d matter?
Yes — a/b must equal c/d, not a/c or any other pairing. Keep the same quantity type in the same position on both sides (e.g. both numerators in cups, both denominators in cookies) or the proportion won't mean anything.
What if one of the ratio terms is zero?
A zero in a denominator position (b or d) makes the ratio undefined — you can't divide by zero — so the calculator will show an error rather than a result in that case.